1. Field of the Invention
The present invention relates to a measurement apparatus, a measurement method, an exposure apparatus, and a device fabrication method.
2. Description of the Related Art
In recent years, a projection optical system (projection lens) mounted in a projection exposure apparatus is required to have a performance as high as a wavefront aberration (transmitted wavefront aberration) of 10 mλ RMS or less (a wavelength λ=248 nm, 193 nm, and so on). This has made it necessary to accurately measure the wavefront aberration of the projection optical system (on the order of about 1 mλ).
It is a common practice to use an interferometer to measure the wavefront aberrations of the projection optical system on a plurality of wavefronts in its field of view. Especially a Fizeau interferometer, for example, allows high-accuracy wavefront measurement by the fringe scanning method because it includes a piezoelectric element which moves, in the optical axis direction of the projection optical system, an optical element (for example, a TS lens) which splits light from a light source into test light and reference light. The projection optical system is adjusted so as to minimize aberration coefficients obtained by polynomial expansion of the wavefronts measured by the interferometer (e.g., expansion in a series of Zernike functions).
Japanese Patent Laid-Open Nos. 2000-277411 and 2002-22608 propose details of this technique.
Unfortunately, a piezoelectric element which performs fringe scanning does not always have a strictly linear response characteristic, so the moving velocity of the optical element often does not become constant (i.e., the movement of the optical element often does not become linear with respect to time) even when the driving voltage applied to the piezoelectric element is changed linearly. If such a change in the moving velocity of the optical element (to be referred to as “fringe scanning nonlinearity” hereinafter) remains until measurement, an error is included in the measured wavefront (i.e., a measurement error occurs).
Also, it is indeed possible to measure the average phase in each bucket during fringe scanning based on electronic moiré to obtain the response characteristic of the piezoelectric element, thereby controlling the driving voltage applied to the piezoelectric element so as to maintain the moving velocity of the optical element constant. However, a commonly-used CCD camera cannot perform fine sampling owing to constraints of, for example, the number of buckets and the frame rate. This makes it impossible to obtain the response characteristic of the piezoelectric element with sufficient accuracy. It is therefore very difficult to control the driving voltage applied to the piezoelectric element so as to maintain the moving velocity of the optical element constant in practice.